Alkylhalodisilanes are well known in the art. For example, methylchlorodisilanes have the general formula Me.sub.6-x Si.sub.2 Cl.sub.x, where Me stands for the methyl group CH.sub.3, and x=1,2,3,4 or 5. These compounds are chemical intermediates useful in forming a wide variety of compounds, particularly those compounds containing silicon-silicon bonds. One use of alkylhalodisilanes is that wherein the reduced disilane is oxidized in a system in which tetramethyl tin is also oxidized, thereby permitting codeposition of a mixed oxide coating on glass.
Six previously developed methods for preparing chloromethyldisilanes begin with a mixture called the "disilane fraction," which is a by-product from the manufacture of methylchlorosilane monomers. This disilane fraction, typically boiling between 150.degree. C. and 160.degree. C., consists mainly of 1,1,2,2 tetrachlorodimethyldisilane and 1,1,2 trichlorotrimethyldisilane. Thus the starting material already contains the Si-Si bond of the desired product, and the synthesis is designed to replace some of the methyl groups with the desired number of chlorine atoms.
A Grignard reaction, e.g., one utilizing methyl magnesium chloride in ether, is a common way to replace chlorines with methyl groups. However, the Grignard reaction applied to the disilane fraction cannot be stopped conveniently at a partial replacement of chlorine by methyl. Thus, the fully methylated product hexamethyldisilane (Me.sub.3 SiSiMe.sub.3) results. This is shown by Kumada and Yamaguchi (1954). A successful partial Grignard reaction was obtained by Kumada et al (1956), by first converting the disilane fraction to its ethoxy derivative with ethanol/ammonia. After the partial Grignard reaction, the product is reconverted to the chloride with acetyl chloride. The overall yield of 1,2 dichlorotetramethyldisilane was only about 30%.
Another approach to the synthesis of these disilanes is to form the hexamethyldisilane by complete Grignard treatment of the disilane fraction itself, and then to partially chlorinate the resulting hexamethyldisilane. A successful synthesis by this approach was developed by Kumada et al. (1956), using sulfuric acid followed by ammonium chloride, giving a yield of 55% after a total reaction period of 2 days. Later workers discovered better chlorinating agents which increased the yield and decreased the reaction time: Sakurai et al. (1966, 1967) found that dry hydrogen chloride in the presence of AlCl.sub.3 catalyst rapidly chlorinated hexamethyldisilane to 1,2 dichlorotetramethyldisilane with an improved yield of 81%. They also found that by using large amounts of aluminum chloride, acetyl chloride could also be used to chlorinate hexamethyldisilane, with 87% yield of 1,2 dichlorotetramethyldisilane. However, a disadvantage of using acetyl chloride is that the subsequent separation of the large amounts of aluminum chloride from the product is difficult. Probably the most satisfactory chlorinating agent previously found for hexamethyldisilane is trimethylchlorosilane, Me.sub.3 SiCl, which Ishikawa et al. (1970) found to give a 93% yield of 1,2 dichlorotetramethyldisilane.
A disadvantage of the methods based on chlorination of hexamethyldisilane is that the major part of the Grignard reagent is wasted, in the sense that most of its methyl groups end up in by-products, rather than in the 1,2 dichlorotetramethyldisilane. Typically, one adds about 3.5 methyl groups per molecule of disilane to form the hexamethyldisilane and then removes 2 of them during the subsequent chlorination. A recent synthesis by Matsumoto et al. (1977) avoids this use of large amounts of Grignard reagent by partial methylation of the disilane fraction using trimethylchlorosilane. The yield is, only about 43% even after 2 successive methylations.
Most of these known synthetic methods have also been adapted to product 1,1,2 trichlorotrimethyldisilane and chloropentamethyldisilane, by changing reactant proportions and/or reaction temperatures, but the same basic disadvantages remain in such analogous procedures.
The following published references are pertinent to the above discussion:
(1) Kumada and Yamaguchi, J. Chem. Soc. Japan, Ind. Chem. Sect. (1954), vol. 57, pp. 175-177. PA0 (2) Kumada et al., J. Org. Chem. (1956), vol. 21, pp. 1264-1268. PA0 (3) Sakurai et al. Tetrahedron Letters (1966), vol. 45, pp. 5493-5497. PA0 (4) Sakurai et al., J. Organometallic Chem. (1967), vol. 7, pp. P15-P16. PA0 (5) Ishikawa et al., J. Organometallic Chem. (1970), vol. 23, pp. 63-69. PA0 (6) Matusomoto et al., J. Organometallic Chem. (1977), vol. 142, pp. 149-153. PA0 (7) Japanese Patent Publication 70/12726 to Kumada, Ishikawa